The present invention relates generally to body tissue fixation systems, including body tissue fixation hardware comprising biocompatible, bioabsorbable (resorbable) thermoplastic plates, and methods of using those systems and hardware.
Traditional orthopedic and traumatological fixation systems to facilitate bone fracture healing (osteosynthesis) typically employ metallic hardware, e.g., plates, screws, rods and the like, formed of biocompatible, corrosion resistant metals such as titanium and stainless steel. Typical metallic plates are described, e.g., in the book, F. Sxc3xa9quin and R. Texhammar, AO/ASIF Instrumentation, Springer-Verlag, Berlin, Heidelberg, 1981, at p. 21-22, 55-79, 107-108, 117-122, the entire discloser of which is incorporated herein by reference. While such systems are generally effective for their intended purposes, they possess a number of inherent shortcomings. For example, metal release to the surrounding tissues has been reported. See, e.g., L.-E. Moberg et al. Int. J. Oral. Maxillofac. Surg. 18 (1989) at pp. 311-314, the entire disclosure of which is incorporated herein by way of this reference. Other reported shortcomings include stress shielding, see P. Paavolainen et al., Clin Orthop. Rel. Res. 136 (1978) 287-293, the entire disclosure of which is incorporated herein by way of this reference, and growth restriction in young individuals, see K. Lin et al., Plast. Reconstr. Surg. 87 (1991) 229-235, the entire disclosure of which is likewise incorporated herein by way of this reference. In infants and young children, there is the risk that metallic plates and screws can sink into and below the cranial bone, as a consequence of skull bone growth, thereby threatening the brain. See, e.g., J. Fearon et al., Plast. Reconstr. Surg. 4 (1995) 634-637, the entire disclosure of which is incorporated herein by way of this reference. Therefore, it is generally recommended that non-functional implants should be eventually removed, at least in growing individuals. See C. Lindqvist, Brit. J. Oral Maxillofac. Surg. 33 (1995) p. 69-70, the entire disclosure of which is incorporated herein by way of this reference.
Especially in maxillofacial and in cranial surgery, metallic mini plates are popular for use. See e.g., W. Muhlbauer et al., Clin. Plast. Surg. 14 (1987) 101-111; A. Sadove and B. Eppleg. Ann. Plast. Surg. 27 (1991) 36-43; and R. Suuronen, Biodegradable Self-reinforced Polylactide Plates and Screws in the Fixation of Osteotomies in the Mandible, Doctoral Thesis, Helsinki University, Helsinki, 1992, p. 16, and references therein, the discloures of which are incorporated herein by reference. Mini plates are small, thin, narrow plates, which have holes for screw fixation. They are typically located on bone, perpendicularly over the fracture to fix the bone mass, on both sides of the fracture to each other. Typical geometries of mini plates are described e.g. in U.S. Pat. No. 5,290,281 at FIGS. 6A-6F, the entire disclosure of which is incorporated herein by way of this reference.
The main advantage of metallic plates (like titanium, stainless steel and cobalt chrome molybdenum plates), is that they are strong, tough and ductile so that they can be deformed or shaped (e.g., bended) at room temperature in the operation room, either by hand or with special instruments, to a form corresponding to the surface topography of bone to be fixed. In this way, the plate can be fixed flush on the bone surface to which the plate is applied.
In light of the above shortcomings of metallic plates, however, bioabsorbable plates have been developed for fracture fixation. Longitudinal, six-hole plates were developed for orthopaedic animal studies. See Eitenmxc3xcller et al., European Congress on Biomaterials, Abstracts, Instituto Rizzoli, Bologna, 1986, p. 94, the entire disclosure of which is incorporated herein by this reference. However, because of their inadequate strength, some of the plates were broken in animal experiments involving fracture fixation.
A special advantage of bioabsorbable plates is that they can be provided with openings for the insertion therethrough of surgical fasteners (like screws), while allowing means to permit the formation of additional fastener openings therethrough during a surgical procedure at the surgeon""s discretion, as has been described in European Patent specification EP 0 449 867 B1, the entire disclosure of which is incorporated herein by way of this reference.
The main disadvantage of prior art bioabsorbable plates is that they can be deformed (bended) permanently and safely only at elevated temperatures above their glass transition temperature (Tg), as has been described e.g. in EP 0.449 867 B1 and in U.S. Pat. No. 5,569,250, the entire disclosures of which are incorporated herein by way of this reference. Below their Tg, the prior art bioabsorbable plates are brittle and break easily when deformed. Only at temperatures above the Tg does the molecular structure of prior art plates have enough mobility to allow shaping (e.g., bending) without the risk of breaking. Accordingly, U.S. Pat. No. 5,569,250 describes a biocompatible osteosynthesis plate that is capable of being used in a secured relationship over a plurality of adjacent bone portions. That biocompatible osteosynthesis plate includes an elongated section having a top face and a bottom face, at least one fastener opening disposed between the top face and the bottom face, and means disposed upon the elongated section to permit the formation of additional fastener openings therethrough, during a surgical procedure. The osteosynthesis plate is in a first thermochemical state in a first configuration and is capable of being converted to a second thermochemical state so that it may be deformed prior to fixation. The first thermochemical state is typically room temperature (operation room conditions) and the second thermochemical state is typically an elevated temperature above the Tg of the polymer material (e.g., for polylactides between 50-60xc2x0 C.). Accordingly, in order to shape the plates disclosed in U.S. Pat. No. 5,569,250, they must be changed from their first thermochemical state to the second thermochemical state by heating, and thereafter they must be changed again back to the first thermochemical state prior to fixation. Because the thermal conductivity of polymeric materials is poor, the conversion of material to a second temperature is a slow process. Therefore, the clinical use of plates of U.S. Pat. No. 5,569,250 is tedious, slow and complex, especially if the surgeon must shape the plate several times to make it fit exactly to the form of the bone to be fixed.
K. Bessho et al., J. Oral. Maxillofac. Surg. 55 (1997) 941-7945, the entire disclosure of which is incorporated herein by reference, described a bioabsorbable poly-L-lactide miniplate and screw system for osteosynthesis in oral and maxillofacial surgery. However, in order to shape the plates of that reference, they first must be heated by immersion in a hot sterilized physiologic salt solution or by the application of hot air until they become plastic, and only then can they be fitted to the surface of the bone.
EP 0 449 867 B1 describes a plate for fixation of a bone fracture, osteotomy, arthrodesis etc., said plate being intended to be fixed on bone at least with one fixation device, like a screw, rod, clamp or corresponding device, wherein the plate comprises at least two essentially superimposed plates to provide a multilayer plate construction. The individual plates of said multilayer plate construction are flexible, so as to permit a change of form of said multilayer plate construction to substantially assume the shape of the bone surface in the operation conditions by means of an external force, such as by hand and/or by bending instrument directed to said multilayer plate construction, whereby each individual plate assumes a position of its own with respect to other individual plates by differential motion along the respecitive surfaces of coinciding plates.
Although the said multilayer plate fits even the curved bone surface without heating of individual plates, the clinical use of multilayer plates is tedious, because the single plates easily slip in relation to each other before fixation. Additionally, the thickness of multilayer plate system easily becomes too thick for cranio maxillofacial applications, causing cosmetic disturbances and increased risks of foreign body reactions.
U.S. Pat. No. 4,671,280, the entire disclosure of which is incorporated herein by reference, describes the manufacturing of a fastener member or staple, by the winding of an oriented bioabsorbable polymeric filament around, a forming bar, which winding is carried out at a temperature below the glass transition temperature of the polymer. Ordinarily, winding will be done at ambient temperature. Because the oriented filament is quite stiff, the coils are bowed out slightly from the sides of the forming bar. Thus, the coils do not fully assume the desired fastener member (or staple) configuration until the filaments are heated, which will normally be done during the annealing step (see, e.g., U.S. Pat. No. 4,671,280; Column 5, first two paragraphs). Thus, while U.S. Pat. No. 4,671,280 may describe some bending of drawn filament at an ambient temperature, the bending does not give the desired configuration of the material until the filaments are additionally heated. The filaments are heated during the annealing step to a temperature above the glass transition temperature of the material (see also Example 1 of U.S. Pat. No. 4,617,280).
A need, therefore, exists for a bioabsorbable (bioresorbable or biodegradable) osteosynthesis device, like a plate, which is thin and substantially rigid and substantially deformable at a first thermochemical state, being also dimensionally stable before and after deformation (shaping) in the said first thermochemical state. A need also exists for a bioabsorbable (bioresobable or biodegradable) osteosynthesis plate, which is strong, tough, does not produce a substantial inflammatory response, and which plate can be deformed, yet dimensionally stable at temperatures below the glass transition temperature (Tg) of the material from which the device is made, to facilitate shaping. A need further exists for such a bioabsorbable (bioresorbable or biodegradable) osteosynthesis plate, which is strong, tough, does not produce a substantial inflamatory response, and which plate can be deformed, yet dimensionally stable at room temperature in operation room conditions, to facilitate the shaping of the plate. Likewise, a need exists for such a bioabsorbable (bioresorbable or biodegradable) osteosynthesis plate, which is strong, tough, does not produce a substantial inflammatory response, and which plate can be deformed, yet dimensionally stable in operation room conditions (in the first thermochemical state) to allow its fixation on bone without distortion of the configuration of the bone fragments to be fixed, and which shaped plate is also dimensionally stable at a second thermochemical, state, in tissue conditions, when fixed on bone surface to facilitate non-problematic bone fracture healing.
Prior art, U.S. Pat. No. 5,569,250, teaches that bioabsorbable polymeric fixation implants, like plates, should be manufactured of non-oriented material and that the implants are relatively rigid at a first thermochemical state and are relatively deformable only at a second thermochemical state (at elevated temperature) to which the implant is temporarily brought prior to implantation.
In this invention we have found, surprisingly, that brittle and/or relatively weak bioabsorbable thermoplastic polymers, copolymers, polymer alloys or composites with ceramic particulate fillers or fiber reinforcements, having Tg of the material above human body temperature, which materials cannot be deformed at room temperature, can be transformed through uni- and/or biaxial orientation of the material in the solid state to materials which are deformable at room temperature. Accordingly, the present invention describes uni- and/or biaxially oriented, rigid and tough materials and implants, like plates, which can be deformed at a first thermochemical state, like at room temperature in operation room conditions, prior to implantation; and which implants retain their deformed (shaped) form well in the second thermochemical state at body, temperature in tissue conditions, when implanted on bone, so that they keep the fixed bone fragments essentially in the desired position to facilitate bone fracture healing.
It should be emphasized that the first thermochemical state can be any temperature below Tg of the material down to the room temperature area, because the uni- and/or biaxially oriented materials retain their properties of being substantially deformable and substantially rigid at such temperatures. An advantage of the present invention is to provide a low profile uni- and/or biaxially oriented biocompatible implant of sufficient strength to be capable of effecting a secured relationship between a plurality of adjacent bone portions. Another advantage of the present invention is to provide an uni- and/or biaxially oriented biocompatible implant that is bioresorbable over a desired period of time while not generating a substantial inflammatory response. A further advantage of the present invention is to provide an uni- and/or biaxially oriented bioabsorbable and biocompatible implant, like a plate, that is relatively rigid at a first thermochemical state, but is also relatively deformable at said first thermochemical state prior to implantation.
A further advantage of the present invention is to provide an uni- and/or biaxially oriented bioabsorbable implant that is capable of being repeatedly deformed at the said first thermochemical state prior to implantation. Another advantage of the present invention is to provide an uni- and/or biaxially oriented biocompatible implant that can be easily and inexpensively manufactured with reduced internal stresses. A further advantage of the present invention is that it provides an uni and/or biaxially oriented biocompatible fixation device that is capable of securing another such uni- or biaxially oriented biocompatible implant device and one or more adjacent bone portions.
The present invention, moreover, in one form thereof, provides a low-profile uni- and/or biaxially oriented biocompatible osteosynthesis plate that is capable of being shaped to secure a plurality of adjacent bone portions. The osteosynthesis plate of the present invention includes an elongated section having a top face and a bottom face, which elongated section is capable of being shaped to traverse a fracture site or osteotomy site for subsequent fixation to adjacent bone portions. The uni- and/or biaxially oriented osteosynthesis plate further includes a plurality of fastener openings disposed between the top face and bottom face to allow the traverse of a plurality of surgical fasteners therethrough. The osteosynthesis plate further includes means disposed upon the elongated section to permit the formation of additional fastener openings therethrough during a surgical procedure, at the discretion of the surgeon. The osteosynthesis plate is relatively rigid at a first temperature and is deformable in three dimensions, yet dimensionally stable, at said first temperature. The osteosynthesis plate retains a deformed position at said first temperature in operation conditions, but can be subsequently returned to its original configuration by redeformation at said first temperature and said first thermochemical state. As such, the uni- and/or biaxially oriented osteosynthesis plate of the present invention may be repeatedly deformed and returned to its original configuration at said first temperature (first thermochemical state), in order to contour the osteosynthesis plate precisely to a desired configuration through successive iterations.
The present invention also includes bioresorbable fixation devices, or bone screws, that are capable of being inserted through fastener openings disposed within the uni- and/or biaxially oriented osteosynthesis plates of the present invention. As such, the present invention contemplates a bone stabilization device including an uni- and/or biaxially oriented bioresorbable osteosynthesis plate and bioresorbable surgical fastener.
The present invention also provides a method for forming a low-profile, uni- and/or biaxially oriented biocompatible osteosynthesis plate, including the steps of formation of a sheet stock, polymer orientation uni- and/or biaxially, formation of an uni- and/or biaxially oriented, osteosynthesis plate from oriented sheet stock, finishing, surface cleaning, sterilization and packaging.
The present invention is also directed to a method for enabling a secured relation between a plurality of adjacent bone portions, including the steps of providing a low-profile, uni- and/or biaxially oriented, biocompatible, osteosynthesis plate, positioning the uni- and/or biaxially oriented biocompatible osteosynthesis plate upon a plurality of adjacent bone portions, providing a plurality of surgical fasteners for enabling a fixed relation between the uni- and/or biaxially oriented osteosynthesis plate and at least one adjacent bone portion, positioning the plurality of surgical fasteners within a plurality of fastener openings upon the uni- and/or biaxially oriented osteosynthesis plate and securing the plurality of surgical fasteners into the adjacent bone portions.
Uni- and/or biaxial orientation of polymers or polymer composites with solid state deformation is a well known process in polymer science and technology. During orientation, polymer molecules or their segments tend to align with their long axis in the orientation direction. A description of molecular background of orientation of polymeric materials and of its physical characterization is given, e.g., in U.S. Pat. No. 4,968,317, the entire disclosure of which is incorporated herein by reference. The effects of orientation are most pronounced in partially crystalline polymers, but it is also possible to orient non-crystalline (amorphous) polymers, as has been described in PCT/FI96/00511; the entire disclosure of which is also incorporated herein by way of this reference.